You need to have your IPLL defined like so:

IPLL, OD.B8.DD:01, test_one, All_Lights|Blah, PLM

The :01 is the switch side of the equation so now you can link in both
directions.

Thanks George.  I tried that and it didn't seem to work.  My mht file is now:
Format = A
INSTEON_PLM,   PLM,
IPLL, 0D.B8.DD:01, test_one, All_lights|Blah, PLM, # 2476S
IPLL, 16.1F.98:01, test_two, All_lights|Blah, PLM, # 2476D

SCENE_MEMBER, test_one, test_two, 100%, 2s
SCENE_MEMBER, test_two, test_one, 100%, 2s

As before, the crosslinking works, but when I touch the paddle on test_one to turn it on, mh only thinks test_one is on and doesn't know that test_two also went on.